I don't know if I qualify as a non, as such, but because I no longer fit the criteria for HPD after having therapy, I'll give my ten cents.
In an ideal world, a child who is loved unconditionally by both parents will develop healthy self esteem, and they will carry this view of themselves into adulthood, being neither narcissistic or having cripplingly low self esteem. They will be able to love themselves, just as they are, faults and all, and won't have a desire to prove themselves to others or base their self esteem upon external things such as other people's opinions of them or on how they appear to others.
A child who has not been so fortunate may have had abusive parents or parents who imposed conditions upon them which they had to adhere to in order to be "loved". This will have had a dramatic effect upon their view of themselves and of course their self esteem. Deep inside, they may even feel an intense hatred for themselves. They may react in various ways in order to compensate for this lack of self esteem. If a child reacts in such a way that they develop HPD, their main focus will be on gaining the approval and attention of others in order to feel validated or worthy. They may act in ways that overcompensate for their lack of self esteem, and this is, in effect a form of narcissism. Because of their narcissistic way of viewing things, other people cease to be viewed as individuals in their own right, and become possible sources of supply, who exist mainly to validate their self worth. Because they are no longer viewed as people in their own right, it is impossible for the disordered person to feel true empathy, which of course would compromise their ability to feel love in the true sense. The person with HPD has lost the sense of who they really are, and they base their identity upon a false self. Other people are seen as extensions of themselves, and are therefore also false,and are not seen as they really are.
By working on the issues that created the HPD in the first place, a person can begin to see themselves differently, and learn who they really are - and not as the false construct that has been built, and by doing so, learn to love themselves.